EP0381658A2 - Incremental measuring system - Google Patents

Abstract

In the incremental length measuring system, a scale (1) with a measuring graduation (3) and an associated scanning unit (2) with scanning elements for generating analog measuring signals and an evaluation unit (9) for generating digital counting signals from the analog measuring signals are provided. To generate a reference pulse, reference marks (12, 13) are provided in pairs with a longitudinal spacing from one another on a support (11) which can be attached to the scale (1). Associated scanning elements (10) are connected to the evaluation unit (8, 9, 15). A reference point (R) can be selected between the reference marks (12, 13) and is defined by its distances (Z₁, Z₂) from these reference marks (12, 13). These distances (Z₁, Z₂) are stored in memories (16), so that the evaluation unit (8, 9, 15) depends on the adjustment direction after passing a reference mark (12 or 13) with the scanning unit (10) in the direction of the selected reference point ( R) triggers the reference pulse generated by its own pulse shaping stage (16) after the path corresponding to the assigned storage value and determined from the scanning of the measuring graduation (3).

Description

The invention relates to an incremental measuring system according to the preamble of claim 1.

From DE-A-34 45 694 an incremental measuring system is known, the scale embodiment of which can be scanned contactlessly via the scanning unit according to optoelectronic scanning principles. But there are also known measuring systems with inductively, magnetically or capacitively scales and corresponding reference marks. When scanning the scale embodiment, periodic, z. B. receive sinusoidal analog signals in which a complete signal train is assigned a complete measuring division increment, which can consist of a bright and a dark field of the measuring division in optoelectronic scanning. At least two measurement signals phase-shifted by 90 ° are generated, in which one leads the other depending on the adjustment direction, so that the adjustment direction can be determined therefrom. When processing the analog measurement signals to digital count signals, an electronic or arithmetic subdivision of the scale is, for. B. possible using microcomputers. The digital count signals can be used as control signals for a machine, for robots and to display the measurement results on display units. In order to establish a clear relationship between the counting signals and the scanned position in each case, it is necessary to set the counting device to specific values, for example zero, at predetermined or preselectable scale. The reference pulses are used for this. It can also be on different longitudinal areas of the scale, but the measuring graduation reference pulses are clearly assigned and reference pulses are generated, it being necessary to define which reference point is just being approached in order to be able to relate the control to the respective scale point. It is known to provide complex control and switching systems for distinguishing between selected and unselected reference points. For precise measurements, the reference points are defined by reference marks attached to the scale together with the measuring graduation, but in a separate track. When they are attached, care must be taken to ensure exact alignment with the scale increments. These reference marks directly generate reference pulses during the manipulation by means of a corresponding grid of a separate scanning unit, which should occur with the measuring and counting signals with the correct edges and phases. Several reference marks can be attached to the scale, one or some of which are selected for actual control. This selection is possible by covering or removing unselected reference marks, which is complex in most encapsulated measuring systems, which may only be carried out by trained specialist personnel and where it is not possible to reapply removed reference marks. On selection systems, mechanical on-off switches are known from DE-A-18 14 785, which are actuated when the scanning unit is adjusted by means of specially provided stops and which activate the evaluation circuit only at selected reference marks. In a construction according to DE-B-25 40 412, these mechanical switches are replaced by magnets which can be attached to preselectable scale positions for actuating selection switches designed as reed relays in the scanning unit. It is also known to provide the reference marks in a plurality of reference tracks, to assign a scanning unit to each track and to determine a reference mark by selecting the scanning unit of the assigned track for the delivery of reference signals. This already requires a complex structure of the scale and the scanning units as well as difficult adjustment work for the activation tion of the measuring system ahead. Formations in which the reference marks from which to select are present in a single track are preferred. To identify a selected reference mark, according to EP-A-0 239 768, the reference marks are applied in the same division and the reference mark adjacent to a selected reference mark is removed. Using a double scanning device with scanning elements attached at the pitch of the reference marks, which are connected in counter-connection, so that signals generated by existing reference marks cancel each other out, it is achieved that at the selected reference mark, when it is scanned because of the distance of the neighboring reference mark, the otherwise a signal generated by this is missing, an evaluable reference signal is generated. Later changes in the selection of the reference marks are practically excluded.

From EP-A-0 172 323 it is known to apply reference mark pairs in a periodically changing division, so that a length coding is produced from which that reference mark half whose reference pulse is evaluated can be defined.

From DD-A-58 394 it is known to apply coding marks in a longitudinally displaceable and lockable manner on a scale, wherein control commands can be triggered by scanning such a coding mark. It is impossible to align a coding mark so precisely with respect to the measuring graduation that the control signals generated, like the reference pulses generated by stationary reference marks, coincide in phase and edge with an analog measuring signal obtained by scanning the measuring graduation or a corresponding digital count signal derived from this analog signal, which can also result in measurement or control errors depending on the direction of approach.

The object of the invention is to provide a measuring system of the type mentioned, in which one or more reference points are determined with low system and circuit complexity and high security, corresponding reference signals can be generated in phase or edge-correct assignment to the measuring or counting signal and if necessary, a change in the location of the reference point or reference points is made possible without significant effort.

The object is achieved by the features of claim 1.

The scale is simplified since only the measuring graduation has to be attached. The reference marks can be designed similarly or identically to the known reference marks, but it is also possible to give them a simpler form, since the only thing that is important is to trigger a ready signal for further evaluation at a precisely defined point when the mark is passed over. The actual reference point is defined by its distances from the reference marks and in itself determined by a counting signal, so that the reference pulse can be triggered in correct phase or edge assignment to the scale increment assigned to the reference point or the measurement signal or counting signal generated by it. Even if the carrier is mounted with reference marks that are not exactly aligned with the measuring graduation with respect to the longitudinal direction, the distance of the reference point from the reference marks can be precisely defined. By using a separate pulse shaper stage to generate the reference pulse, the signal shape or the signal curve of this pulse can be adapted to the needs of the measuring or control device to which it is subjected, so it is completely independent of the signal shape of the signals occurring in the scanning elements when the reference marks are scanned . It is possible to use a pulse shaping stage that determines the pulse shape of the reference pulse Memory. Changes in the position of a reference point to the scale can be carried out by adjusting the carrier in the longitudinal direction of the scale. The carrier can be glued to the scale, screwed down or held in place with its own support device. According to another possibility, the carrier forms a clamp encompassing the scale in the edge region, which can be inserted and adjusted if necessary through the slot of a tube housing which receives the scale and the scanning unit and is sealed by means of sealing lips. For the first attachment, you can first couple a clamp holding onto the scale with an operating sword for the scanning unit, move it with the sword and release this coupling when the point on the scale selected for the reference point is reached.

Identical reference marks can be provided on the carrier, the signals of which remain the same whether they are run over from the right or from the left. Protective measures are provided to prevent an incorrect reference pulse in the memory distance to the left of the left or right of the right reference mark (and not between the reference marks) from being generated when the measuring system is first activated and when the scanning unit is located above the carrier. In the simplest case, the scanning unit for the reference marks, as long as it is located between the reference marks and scans the carrier, generates a signal which differs from that which occurs when the free scale area is scanned. It can also be provided that a reference pulse may only be taken into account after a reference mark has been passed and the direction has been reversed, up to which at least one distance corresponding to the reference mark distance has been covered. Finally, it is also possible to affix marks on the outside of the scale support or protective housing, which indicate the location of the plate and the scanning unit when the system is started, to a location outside of this mounting area poses. One can also provide different reference marks in the signal generation characteristic and make the triggering of the reference pulse dependent on the fact that one reference mark has to be driven over in the direction of the other reference mark beforehand and the path recorded in the memory is covered in this direction. This possibility is offered by the fact that the reference marks do not have to be designed for the generation of identical signals.

A possibility of selecting between the reference points defined by reference mark pairs attached to a plurality of carriers is specified in claim 4.

Further details and advantages of the subject matter of the invention can be found in the following description of the drawings.

In the drawing, an exemplary embodiment is shown in a schematic representation of a part of a scale with measuring graduation which can be scanned according to optoelectronic scanning principles, an attached reference mark carrier and an associated scanning unit, the essential parts of the evaluation unit also being illustrated in the block diagram.

A z. B. from a vitreous scale body 1 can be accommodated together with a along the scale scanning unit 2 in a tubular protective housing, not shown, wherein the scanning unit 2 can be adjusted with the aid of an inserted through sealing lips into a slot in the tubular housing sword that z . B. on a machine tool connects to a tool slide.

A scale scale 3, which can be scanned according to optoelectronic scanning principles, is attached to the scale body. For this scale embodiment, four scanning units can be provided in the scanning unit, each consisting of a scanning grating and a photoelectric receiver. The scanning grids are offset from one another by divisional fractions of the scale embodiment and the associated receivers are connected in pairs in counter-connection, so that at two connections 4, 5, when measuring the measuring graduation, phase-shifted, in the basic form, sinusoidal analog measurement signals occur via lines 6 , 7 a direction detection stage 8 and an evaluation stage 9 are supplied. Separate lighting devices (such as photodiodes) or a common lighting device illuminating the measuring graduation 3 and the scale track assigned to the receiver 10 can be provided for the photoelectric receiver mentioned and a photoelectric receiver 10 to be described. Scanning is possible in the reflected light, transmitted light or with a preferably mirrored scale back side using the reflection light method.

The evaluation unit 9 receives the measurement signals and a signal indicating the adjustment direction of the scanning unit 2 from the direction discriminator 8. During the evaluation, multiplier circuits, e.g. B. potentiometer circuits, for electronic subdivision of the scale or one uses - with then possibly pulsed lighting device for the photoelectric receiver on the scanning grids - multiplier circuits with phase shift detection. A preferred possibility consists in the combination of counter circuits for the measurement signals converted into digital counter signals and a computer for arithmetic interpolation calculation. Especially when using a computer, free running counters can be used for the two counting directions (right and left or "up" and "down") Det, which are periodically queried by the computer, the control or display zero point, which can be defined by a reference pulse, defined by storing the current counter reading at the zero point and the current position of the scanning unit with respect to this defined zero point from the current counter readings and those mentioned Stored values is calculated. The measurement result is shown on a display and / or used for machine and robot controls.

As far as the circuit has been described so far, it is known prior art. According to the invention for generating a or each required reference pulse (which, however, does not have to be used each time the scale point concerned is passed and is usually activated only after a separate switch actuation or for calibration at the start of a measurement or a working day) is a on the scale body 1 attachable, e.g. B. formed as a spring clip, plate 11 is used, which carries two via the unit 10 and interposed grid od. Like. Reference marks 12, 13 can be scanned. The plate can also be pressed onto the scale on its own brackets or glued to the scale. When driving over each of the two scanning marks 12 and 13 generates a signal in the associated photoelectric receiver 10, a characteristic point, for example a flank, of which can be precisely discriminated. These signals are fed via a line 14 to an evaluation stage 15, which is shown separately, but, like the directional discriminator 8, can be part of the evaluation unit 9. This also applies to the memory 16 to be mentioned in connection with the evaluation stage 15. The evaluation stage 15 is connected (or part of it) to the evaluation unit 9 and receives signals indicating the adjustment direction from the direction discriminator 8 and counting signals from the unit 9.

A reference point R is defined by its distance from the reference marks 12, 13.

The reference marks 12, 13 are located at a distance Z. The distance of the reference point R from the left mark 12 is denoted by Z₁ and the distance from the right mark 13 by Z₂. When the evaluation circuit for the reference pulse is activated, when the marker 12 is passed to the right, the evaluation stage 15 is activated, so that it now receives counting pulses or evaluates counting pulses from the evaluation unit depending on the direction of adjustment. In the memory 16 the distances Z₁ and Z₂ corresponding memory values are recorded. The memory Z₁ and Z₂ can also consist of counting circuits which, when the reference signal occurs at 12 or 13 depending on the direction of adjustment, are set to the value Z₁ or Z₂ and are controlled by further counting pulses in the sense of counting down. Since counting pulses are used which are derived from the main measuring system, these pulses are in the edge or phase direction to the analog measuring signal or digital counting signal. As soon as the stored value Z 1 is reached when activated by the mark 12 or the stored value Z 2 (when activated by the mark 13), the evaluation stage 15 emits a reference pulse via a line 17. The reference pulse is always output at the precisely defined point R. The signals generated at the marks 12, 13 can be evaluated in such a way that they only serve to provide the device 15, which in the ready state is further controlled by the counting signals. The distances Z₁ and Z₂ are therefore also defined by whole counting steps.

The following conditions apply when determining the position of the reference point R:
Z = Z₁ + Z₂ and
Z₁ ± n = Z₂, where n can be zero or an integer. In most cases, n becomes as small as possible or with zero choose. The distance Z can be kept very small if necessary. It is necessary in order to be able to precisely define the reference point and to ensure that the scanning unit 10 receives separate signals from 12 and 13 when it passes over it. Possibilities of determining the direction by different design of the two reference marks, by generating a basic level at 10 with the scanning unit located above the plate 11 and initial activation of the reference pulse or activation of the system were mentioned in the previous general description.

At least theoretically, it is possible to provide only one reference mark 12 on the carrier 11 or to mount two scanning units 10 at a distance Z on the scanning unit 2 for the reference marks 12, 13, in the latter case the reference pulse evaluation only taking place if a signal from both scanning units occurs simultaneously . If R is assigned to the measuring or counting signal with the correct phase or edge, i.e. the reference pulse is only triggered by the counting step or one of the following counting steps following the signal derived from the reference mark (s), two defined reference points R are obtained, each according to the adjustment direction of the scanning unit 2. The distance between these reference points can be taken into account in that in one adjustment direction the reference pulse obtained is used in a conventional manner and in the other adjustment direction the main counter 9 when the reference pulse occurs at the second reference point is adjusted by the distance of the corrected value is set at both reference points.

Claims (4)

1. Incremental measuring system, in particular a length measuring system, with a scale (1) on which a measuring graduation representing a non-contact scannable scale is provided, a scanning unit (2) with scanning elements (4, 5) for generating analog measuring signals from the scanning of the measuring graduation ( 3) and an evaluation unit (9) for generating digital counting signals from these measuring signals, reference marks (12, 12) being provided in association with the measuring graduation (3), for which scanning elements (10) and an evaluation circuit () in the scanning unit (2) 15) are provided which, depending on the signals occurring during the scanning of the reference marks (12, 13), generate a reference pulse associated with a scale point defined by their distance from a reference mark, characterized in that the reference marks (12, 13) are paired with longitudinal distances from one another a carrier (11) attachable to the scale (1) at a selectable position ht are that a reference point (R) between these reference marks (12, 13) can be selected and defined by its distances (Z₁, Z₂) from the two reference marks and that these distances (Z₁, Z₂) are recorded in memories (16), so that the evaluation unit (15), depending on the direction of adjustment, after passing over one of the two reference marks (12, 13) with the scanning unit (2) in the direction of the selected reference point (R) after the storage value corresponding to the assigned storage value has been covered, from the scanning of the measuring graduation (3 ) determined path triggers the reference pulse generated by its own pulse shaper stage (16). 2. Measuring system according to claim 1, characterized in that the evaluation unit (8, 9, 15) in phase or flank correct assignment to the analog measuring signal generated at the reference point (R) associated scale increment or triggers the digital count signal derived from this. 3. Measuring system according to claim 1 or 2, characterized in that the pulse shaper stage (15, 16) has a memory (16) determining the pulse shape of the reference pulse. 4. Measuring system according to one of claims 1 to 3, characterized in that when using a plurality of reference points, the individual reference points are characterized by mutually different reference mark pairs in the signal generation characteristic on the assigned carriers which can be attached to the scale and which generate different signals in the scanning unit.

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